It is known to construct Thin Film Bulk Acoustic Wave Resonators (FBARs) on semiconductor wafers including those which are comprised of Silicon (Si) or gallium arsenide (GaAs). For example, in an article entitled “Acoustic Bulk Wave Composite Resonators”, Applied Physics Lett., Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layer of Zinc-Oxide (ZnO) sputtered over a thin membrane of Silicon (Si).
Unfortunately, semiconductor materials have high conductivities and high dielectric permittivity characteristics. These characteristics can have a deleterious effect on piezoelectric coupling efficiency and resonator quality factors, as is described in an article entitled “Temperature Compensated High Coupling and High Quality Factor ZnO/SiO2 Bulk Wave Resonators on High Resistance Substrates”, IEEE Ultrasonics Symp., 1984, pp. 405-410, by T. Shiosaki, T. Fukuichi, M. Tokuda, and A. Kawabata.
It is known to reduce ohmic losses exhibited by semiconductor wafers by using semi-insulating semiconductor wafers, as is evidenced by an article entitled “Coplanar Waveguides and Microwave Inductors on Silicon Substrates”, IEEE Trans. Microwave Theory Tech., vol. 43, no. 9, pp. 2016-2021, 1995, by Adolfo C. Reyes, Samir M. El-Ghazaly, Steve J. Dorn, Michael Dydyk, Dieter K. Schroder, and Howard Patterson. However, the use of these types of wafers necessitates the use of expensive special grade materials, and does not eliminate the presence of stray capacitances.
Additionally, semiconductor wafers and crystalline wafers need to be carefully polished after being cut from a crystal in order to smooth their surfaces. The polishing process can be expensive.
It would be advantageous to provide a substrate that is formed of a low cost material and which exhibits a low permittivity characteristic and low parasitic capacitances. It would also be advantageous to provide a substrate that is formed of a material that does not need to be polished in order to smooth its surfaces.
It is known to construct so called “bridge” structures on FBAR substrate surfaces using a sacrificial layer that is formed of zinc-oxide (ZnO), as is evidenced by an article entitled “An Air-Gap Type Piezoelectric Composite Thin Film Resonator”, IEEE Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara. Similarly, in an article entitled “Multi-layered Ultrasonic Transducers Employing Air-Gap Structure”, IEEE Trans. Ultrason. Ferroelec. Freq. Control, vol. 42, no. 3, May 1995, by Susumu Yoshimoto, Masamichi Sakamoto, Ken-ya Hashimoto, and Masatsune Yamaguchi, a multi-layered ultrasonic transducer is disclosed which includes an air gap formed by the removal of a “sacrificial” ZnO layer.
During the fabrication of these types of FBARs, a sacrificial layer of ZnO are deposited (e.g., sputtered) over a substrate. The sacrificial layer is later removed via an etching step that is performed after all of the layers of the FBAR have been completely formed. One drawback of this process is that the steps of sputtering and forming the sacrificial layer can be tedious and time-consuming. This is because ZnO is a ceramic material, and thus is brittle and has a low thermal conductivity. If, by example, very high power is employed during the sputtering of the ZnO, the “target” substrate may break. Also, the growth rate of the ZnO must be controlled to produce a correct crystal orientation and crystallite size distribution. Thus, the growth rate may need to be limited to only 2 μm/h.
Another drawback of using a sacrificial layer formed of ZnO is that the surface of the crystalline ZnO film is textured, and thus causes acoustic energy scattering losses to occur within the FBAR. Also, the textured surface of the ZnO may cause the surface of a layer (e.g., the bridge layer) formed over the sacrificial ZnO layer to become deformed. A further drawback of employing a sacrificial layer of ZnO is that during the etching of the layer to form an air gap, a piezoelectric ZnO layer that is formed on the sacrificial layer can become damaged.
In view of these problems, it can be appreciated that it would be advantageous to provide a method for fabricating an FBAR using a sacrificial layer that is formed of a material having characteristics that are more beneficial than those of ZnO and other materials conventionally used to form sacrificial layers.